% File src/library/base/man/lengths.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2023 R Core Team
% Distributed under GPL 2 or later

\name{lengths}
\alias{lengths}
\title{Lengths of List or Vector Elements}
\description{
  Get the length of each element of a \code{\link{list}} or atomic
  vector (\code{\link{is.atomic}}) as an integer or numeric vector.
}
\usage{
lengths(x, use.names = TRUE)
}
\arguments{
  \item{x}{a \code{\link{list}}, list-like such as an
    \code{\link{expression}}, \code{NULL} or an atomic vector (for which
    the result is trivial).}
  \item{use.names}{logical indicating if the result should inherit the
    \code{\link{names}} from \code{x}.}
}
\details{
  This function loops over \code{x} and returns a compatible vector
  containing the length of each element in \code{x}.  Effectively,
  \code{length(x[[i]])} is called for all \code{i}, so any methods on
  \code{length} are considered.

  \code{lengths} is generic: you can write methods to handle
  specific classes of objects, see \link{InternalMethods}.
}
\note{
  One \I{raison d'\enc{être}{etre}} of \code{lengths(x)} is its use as a
  more efficient version of \code{sapply(x, length)} and similar
  \code{*apply} calls to \code{\link{length}}.  This is the reason why
  \code{x} may be an atomic vector, even though \code{lengths(x)} is
  trivial in that case.
}
\value{
  A non-negative \code{\link{integer}} of length \code{length(x)},
  except when any element has a length of more than
  \eqn{2^{31}-1}{2^31 - 1} elements, when it returns a double vector.
  When \code{use.names} is true, the names are taken from the names on
  \code{x}, if any.
}
\seealso{
  \code{\link{length}} for getting the length of any \R object.
}
\examples{
require(stats)
## summarize by month
l <- split(airquality$Ozone, airquality$Month)
avgOz <- lapply(l, mean, na.rm=TRUE)
## merge result
airquality$avgOz <- rep(unlist(avgOz, use.names=FALSE), lengths(l))
## but this is safer and cleaner, but can be slower
airquality$avgOz <- unsplit(avgOz, airquality$Month)

## should always be true, except when a length does not fit in 32 bits
stopifnot(identical(lengths(l), vapply(l, length, integer(1L))))

## empty lists are not a problem
x <- list()
stopifnot(identical(lengths(x), integer()))

## nor are "list-like" expressions:
lengths(expression(u, v, 1+ 0:9))

## and we should dispatch to length methods
f <- c(rep(1, 3), rep(2, 6), 3)
dates <- split(as.POSIXlt(Sys.time() + 1:10), f)
stopifnot(identical(lengths(dates), vapply(dates, length, integer(1L))))
}
\keyword{attribute}
